DNA Structure & Replication

Question 1

What are DNA nucleotides made up of?

Answer 1

Phosphate group
Deoxyribose
Nitrogenous base (Adenine, Thymine, Guanine, Cytosine)

Question 2

How many hydrogen bonds do G/C form?

Answer 2

3 hydrogen bonds

Question 3

How many hydrogen bonds do A/T form?

Answer 3

2 hydrogen bonds

Question 4

How is the DNA chain held together?

Answer 4

Phosphodiester bonds form between 5’ phosphate groups and 3’ - OH groups on adjacent nucleotides.
Forming the sugar-phosphate backbone

Question 5

What is chromatin?

Answer 5

DNA that is coiled around histone proteins.

more active genes = less tightly coiled

Question 6

Why is DNA replication called semi-conservative?

Answer 6

Each strand of the double helix acts as a template for a new strand.
Meselson Stahl experiment

Question 7

Describe the process of DNA replication at the ‘replication fork’.

Answer 7

1. DNA helicase uses ATP to break the H bonds between complementary strands.
2. Single-strand binding proteins prevent the strands from re-joining.
3. On the ‘leading strand’ DNA is synthesised continuously.
4. On the ‘lagging strand’ DNA primase synthesises short RNA primers (provide a free 3’-OH) to kick off DNA synthesis.
5. DNA polymerase III carries on the chain from the primers, creating separate fragments of DNA called Okazaki fragments.
6. DNA polymerase I then replaces the RNA primers with DNA.
7. DNA ligase seals the gap between the fragments.

Question 8

How often do errors occur in the addition of nucleotides?

Answer 8

1 in every 100,000.
Polymerases can edit the growing strand backwards (3’ to 5’), reducing error frequency to 1 in 107.
Further checks by other enzymes reduces this to 1 in 109.

Question 9

What can cause mutations in DNA sequence?

Answer 9

Error
Radiation
Chemicals (mutagens e.g. carcinogens)

Question 10

What does EMS do?

Answer 10

Changes Guanine to Thymine.

Causing GC -> AT

Question 11

What types of mutation are there?

Answer 11

Deletion
Insertion
Substitution
Rearrangement

Question 12

What do Base excision repair proteins do?

Answer 12

Cut out specific damaged bases.

Very specific to type of damage.

Question 13

What do Nucleotide excision repair proteins do?

Answer 13

Cut out sections of the damaged strand.

Less specific.

Question 14

How is damaged DNA replaced?

Answer 14

DNA polymerase I copies the intact strand and replaces the damaged DNA which has been removed by excision repair proteins